Sulfuric acid concentration sensor for lead storage battery

ABSTRACT

A sulfuric acid concentration sensor for a lead storage battery comprising a quartz resonator changing its characteristic frequency in a manner of single-valued function according to the change of sulfuric acid concentration and an oscillation circuit (20) oscillating the quartz resonator, the quartz resonator is immersed in an electrolyte (6) so as to be oscillated, and a characteristic frequency of the quartz resonator at this moment is obtained so as to determine the sulfuric acid concentration. The sensor of this invention is compact in its size, simple in its structure and cheap in its cost, so that it can be applied to a lead storage battery for automobile.

TECHNICAL FIELD

This invention relates to a sensor for detecting a sulfuric acidconcentration of an electrolyte for a lead storage battery.

BACKGROUND ART

The greater part of a lead storage battery is used for starting anautomobile. When a charging condition of the lead storage battery is notsufficient, the battery is short of its residual capacity so that theautomobile becomes unable to be started. If the charging condition isknown previously, countermeasures such as execution of supplementarycharging etc. may be taken before stopping the automobile. However, ithas been impossible to know the charging condition in advance, such thatmuch inconvenience has been experienced. Charging of the lead storagebattery has been done by supplying electric power from a generator, anda supply voltage has been controlled by a regulator to a constant valueso as to avoid an overcharging. However, the voltage of the lead storagebattery changes delicately depending on a discharge quantity, atemperature, a frequency of charge and discharge, and the history of thebattery etc. For this reason, the overcharging of the battery can not beavoided by only a control of the voltage through means of the regulatori.e. only a control of voltage determined on the electric circuit side.This has been a major cause of the short service life of the leadstorage battery for an automobile.

From the reasons as mentioned above, it has been strongly demanded todetect a charging condition or discharging condition of the lead storagebattery.

As a method for detecting the charge and discharge conditions of leadstorage battery, systems utilizing the following four methods formeasuring a concentration of sulfuric acid forming an electrolyte havebeen known. However, any system has been expensive and not practical foruse in the lead storage battery for automobile.

(1) Refractive index measuring method

A system of this method is composed of a light emitting diode, a lightreceiving diode and an optical path. This is a method for measuring asulfuric acid concentration by making use of a property of sulfuric acidforming the electrolyte to change its refractive index according to itsconcentration. This method has already been put in practical use for astationary lead storage battery. However, it has been impossible tominimize a size and reduce a cost because of necessity for executing aphotoelectric conversion and preventing the optical path from beingcontaminated. For this reason, this method is not used for the leadstorage battery for automobile.

(2) Specific gravity measuring method

This is a method for measuring a specific gravity of sulfuric acid byusing a float. This is an inexpensive and easy method. In addition, thisis a very useful and certain method in a manual operation such asmaintenance. However, this method includes difficulties in respect ofcost and structure in order to transmit measured data as electricsignals to a data processor located at a center of automobile.

(3) Electrochemical method

This is a method in which an electrode system for sensor comprisingcomponents of metal, sulfuric acid and metal oxide is separatelyinstalled, and the concentration is measured utilizing such a propertythat an electromotive force depends on the sulfuric acid concentration.However, an appropriate electrode for this purpose can not be obtainedyet. Only one put in practical use is an electrode system comprisingcomponents of lead, sulfuric acid and lead dioxide. However, this systemis not appropriate for practical use because periodic reproduction ofboth electrodes is required.

(4) Electric conductivity method

This is a method for measuring an electric conductivity of sulfuricacid. However, since the electric conductivity of sulfuric acidconcentration becomes a maximum in a condition where about a quarter ofthe sulfuric acid is discharged, the sulfuric acid concentration can notbe determined from the electric conductivity unequivocally. In addition,it is required to process data in consideration of various factors suchas stirred condition of electrolyte and temporary fluctuation due toexternal electric noise etc., so that this method is very complicated,expensive and lacks in reliability.

Further, the sulfuric acid forming the electrolyte of lead storagebattery is strongly acidic, and an inside of battery is under a verystrong oxidation-reduction atmosphere. For this reason, there are verymany limitations in materials

This invention is made in consideration of the above-mentioned problems,and an object of this invention is to provide a sulfuric acidconcentration sensor appropriate to a lead storage battery for anautomobile which is small in its size, simple in its structure and cheapin its price.

DISCLOSURE OF THE INVENTION

This invention provides a sulfuric acid concentration sensor for a leadstorage battery comprising a quartz resonator changing itscharacteristic frequency in a manner of single-valued function accordingto a change of sulfuric acid concentration and an oscillation circuitoscillating the quartz resonator, the quartz resonator being immersed inan electrolyte so as to be oscillated, and a characteristic frequency ofthe quartz resonator at this moment being obtained so as to determinethe sulfuric acid concentration.

When the quartz resonator is oscillated by the oscillation circuit, thecharacteristic frequency of the quartz resonator is changed by areaction from the electrolyte. Since this change is in a single-valuedfunctional relation with a change of a sulfuric acid concentration ofthe electrolyte, the sulfuric acid concentration can be determined byobtaining the characteristic frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially sectional vertical view showing a lead storagebattery for automobile adopting a sulfuric acid concentration sensor ofembodiment 1. FIG. 2 is an enlarged partially sectional vertical view ofa port plug of embodiment 1. FIG. 3 is a partially enlarged view of FIG.2. FIG. 4 is a circuit diagram showing an oscillation circuit ofembodiment 1. FIG. 5 is a circuit diagram showing an interface circuitof embodiment 1. FIG. 6 is a diagram showing results obtained bymeasuring plural electrolytes having different sulfuric acidconcentrations by using the sulfuric acid concentration sensor ofembodiment 1. FIG. 7 is a partially sectional vertical view showing alead storage battery for automobile adopting a sulfuric acidconcentration sensor of embodiment 2. FIG. 8 is an enlarged partiallysectional vertical view of a port plug of embodiment 2. FIG. 9 is asectional view taken on a line IX--IX of FIG. 8. FIG. 10 is an enlargedpartial view viewed in a direction of arrow X of FIG. 8. FIG. 11 is acircuit diagram showing an oscillation circuit of embodiment 2. FIG. 12is a circuit diagram showing an interface circuit of embodiment 2. FIG.13 is a circuit diagram showing an interface circuit of another example.FIG. 14 is a circuit diagram showing an oscillation circuit of anotherexample.

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1)

FIG. 1 is a partially sectional vertical view showing a lead storagebattery for an automobile adopting a sulfuric acid concentration sensorof this embodiment. In this figure, 1 denotes a container body, and 2denotes a cover. A plate 3, along with other plates, are housed in thecontainer body 1 in a state of immersion in a sulfuric acid 6 forming anelectrolyte. Reference 6a indicates a liquid surface of the sulfuricacid 6, and 6b is a lower limit line of the liquid surface of thesulfuric acid 6. A filling port 4 is formed on the cover 2, and a poreplug 5 is inserted in the filling port 4 so as to plug it. A sulfuricacid concentration sensor of this embodiment is installed in the portplug 5. This lead storage battery offers a voltage of 12 V and acapacity of 35 Ah, and is divided into six chambers with respectivevoltages of 2 V each. The sulfuric acid concentration sensor isinstalled in each chamber.

FIG. 2 is an enlarged partially sectional vertical view of the born plug5. The sulfuric acid concentration sensor of this embodiment is equippedwith a quartz resonator plate 10, an oscillation circuit 20 and aninterface circuit 30. The quartz resonator plate 10 is generally a discsmaller than or equal to 10 mm. The oscillation circuit 20 and theinterface circuit 30 are assembled in one integrated circuit so as to beincorporated in a cover portion 51 of the port plug 5. Reference 11denotes a holder tube extending perpendicularly from the cover portion51, and the quartz resonator plate 10 is installed in a horizontalposition so as to plug a bottom opening of the holder tube 11. Thequartz resonator plate 10 is stuck to the holder tube 11 by means of anadhesive agent such as an ultraviolet setting resin for example, and theholder tube 11 is thereby sealed to be protected from the sulfuric acid6 which will enter its inside. The length of the holder tube 11 ispreset such that the quartz resonator plate 10 is positioned at thelower limit line of the liquid surface of the electrolyte. As shown byFIG. 3 which is an enlarged partial view of FIG. 2, electrodes 12 and13, composed of conductive films, are attached to both surfaces of thequartz resonator plate 10 so as to form the quartz resonator operatingas an electro-mechanical resonator having a natural oscillating mode.The electrode 12 is attached to an inside surface (upper surface) of thequartz resonator plate 10, and the electrode 13 is attached from anoutside surface (lower surface) through to the inside surface. The endsof the lead wires 14 and 15 are stuck to the electrodes 12 and 13 bymeans of conductive adhesive agents 14a and 15a. Other ends of the leadwires 14 and 15 are connected to the oscillation circuit 20. The quartzresonator plate 10 contacts the sulfuric acid 6 at its electrode 13 sidesurface only. The cover portion 51 is the same as that of a conventionalport plug, so that the sulfuric acid concentration sensor of thisembodiment is installed utilizing the conventional port plug as it is.

A crystal forming the quartz resonator plate 10 comprises a singlecrystal of silicon dioxide having a piezo-electric property. Crystalplates cut out at various angles relative to a crystal axis form theabove-mentioned quartz resonator except for a crystal plate having aplane perpendicular to the Z-axis. For example, a crystal plate having aplane perpendicular to the X-axis produces a longitudinal vibration in adirection of the plate thickness, and a crystal plate having a planeperpendicular to Y-axis or an intermediate direction between the Y-axisand the Z-axis produces a shear vibration in the direction of the platethickness. Among the crystal plates producing the shear vibration, thosehaving a plane perpendicular to the Y-axis are called a Y-cut, and thosehaving a plane in an intermediate direction between the Y-axis and theZ-axis and perpendicular to a direction of 55°45" inclined from theZ-axis are called an AT-cut. It is known that the characteristicfrequency of the quartz resonator plate exhibits respectivecharacteristic temperature coefficients depending upon the cut angle. Asa method for indicating the cut angle of the quartz resonator plate,there is a method of indication by arranging a thickness axis, a lengthaxis, a rotation axis (length axis 1 or width w) and a rotation angle,in this order. According to this method, a Y-cut is indicated as (yxl)0° and an AT-cut is indicated as (yxl) 35°15". A resonator plate havingan optimum temperature coefficient, i.e. a temperature coefficient whichcompensates for a change in a specific gravity and a viscosity ofsulfuric acid as described later, is selected from among those producingthe shear vibration and is then used for the quartz resonator plate 10of this embodiment. Further, a thickness of the quartz resonator plate10 is determined by conversely obtaining it from a desired frequencybecause a characteristic frequency is related to a thickness of theplate.

The holder tube 11 is made of an insulating material provided with aresistance against the sulfuric acid, such as glass, ceramics andplastic etc. and having a coefficient of linear expansion as close tothat of the quartz resonator plate 10 as possible. This is because of aconsideration that the coefficient of linear expansion has an influenceupon the characteristic frequency of the quartz resonator plate 10through the deformation.

The electrode 13 is made of a material such as gold or tin oxide whichdoes not change its property even when it contacts the sulfuric acid. Inorder to carry out a fine adjustment of the characteristic frequency, itis preferable to additionally deposit a proper amount of silver etc.onto the electrode 12 at a side not contacting the electrolyte.

FIG. 4 is a circuit diagram showing the oscillation circuit 20.Reference 21 denotes an operational amplifier. In this circuit 20, theelectrode 13 is in a grounded condition through a large capacitycondenser 60 in respect of alternating current. For this reason, theoscillation circuit 20 has a characteristic that it can be oscillatedand operated even if the electrode 13 is changed to be put under avoluntary direct current potential. This brings about such merits that apower of the oscillation circuit 20 can be supplied directly from thelead storage battery which is an object of measurement, and the sulfuricacid concentration sensor of this embodiment can be applied to any oneof several serial chambers of the lead storage batteries which areserially-connected, i.e. have direct current potentials different fromeach other.

The interface circuit divides, or adds and subtracts, a frequencyobtained by the oscillation circuit as mentioned above, or converts afrequency to voltage or converts a frequency to current; so as to ease atransmission to a distant place, improve a temperature coefficient ofthe measured value, or make the frequency conform to that of thereceiving system at a place to which the frequency is transmitted. Theinterface circuit has a composition of circuit components as shown byFIG. 5. Namely, the interface circuit 30 is designed to compensate thetemperature coefficient of the characteristic frequency of the quartzresonator, and at the same time, to extract only the amount of change inconcentration of the electrolyte and to minimize a value itself of thefrequency, by passing output frequencies of the two differentoscillation circuits 20 through a frequency subtracting circuitcomprising data latches 31 and 32. An output of the data latch 32 is fedto a line driver 33 in order to increase the noise-resistance duringtransmission.

The function of the interface will be described hereunder. When thequartz resonator comprising the quartz resonator plate 10 and theelectrodes 12 and 13 is made to oscillate by the oscillation circuit 20under a state of immersion in the sulfuric acid 6, the quartz resonatorexcites transverse waves in the sulfuric acid 6 through means of itsviscosity, and the sulfuric acid 6 becomes a load of the quartzresonator as its reaction so that the characteristic frequency of thequartz resonator is lowered. It has been proved that the amount ofdecrease in the frequency is theoretically in proportion to a squareroot of a product of a specific gravity and a coefficient of viscosityof the sulfuric acid 6. In this instance, the amount of decrease in thefrequency is influenced by changes of a specific gravity and a viscosityof the sulfuric acid 6 caused by the temperature change. The amount ofdecrease is about -3 ppm/°C. when the sulfuric acid concentration is10%, and about -5 ppm/°C. when it is 39%, within a temperature rangefrom 0° C. to 50° C., so that a temperature rise results in an increaseof the frequency. In this embodiment, in order to positively allow thequartz resonator plate 10 i.e. the quartz resonator itself, to have atemperature coefficient compensating the influence caused by theforegoing temperature change, a quartz resonator plate which includes acut angle θ ranging from 35°40" to 36°10" indicated by (yxl) θ, is usedas occasion demands. This is by utilizing the fact that thecharacteristic frequency decreases at a rate of -5.15 ppm/°C. per 1°increase of cut angle in the vicinity of AT-cut. Thereby, an errorcaused by the temperature change in the measurement of the sulfuric acidconcentration is reduced.

As for other temperature compensation methods, such methods can beconsidered as (1) methods of using a temperature compensation typecondenser as the condenser 14, and (2) methods of using a temperaturesensor such as a thermistor and compensating by a separately installedmicro-computer on the basis of its signal.

FIG. 6 is a diagram showing the results obtained by measuring pluralelectrolytes having different sulfuric acid concentrations by using thesulfuric acid concentration sensor thus constructed. As is obvious fromthis diagram, the frequency of the quartz resonator is approximately inlinear relationship to the sulfuric acid concentration. Therefore, whenthe frequency of the quartz resonator is obtained, the sulfuric acidconcentration can be determined.

In the sulfuric acid concentration sensor thus constructed, theelectrode 13 at the side contacting the sulfuric acid 6 of the quartzresonator plate 10 is grounded with respect to the alternating current,so that it can operate at any direct current potential. Therefore, theoscillation circuit 20 is able to oscillate normally, so that it is ableto measure the sulfuric acid concentration of electrolyte in any one ofseveral serial chambers of the lead storage battery, each of which is atdifferent potentials.

Since the holder tube 11 is made of a material having a coefficient oflinear expansion as close to that of the quartz resonator plate 10 aspossible. there is no chance for the coefficient of linear expansion toinfluence the characteristic frequency of the quartz resonator plate 10through the deformation. From this point again, the sulfuric acidconcentration can be measured correctly.

Further, the quartz resonator plate 10 is positioned at the liquidsurface lower limit line 6b of the electrolyte. Therefore, when aquantity of the electrolyte decreases to a level lower than the lowerlimit line 6b, the quartz resonator plate 10 will be exposed up abovethe liquid surface, and the oscillation frequency becomes abnormallyhigh so that the measured value becomes abnormal. Consequently, thissensor also functions as a level gauge by detecting the abnormal value.

Moreover, the oscillation circuit 20 and the interface circuit 30 areincorporated in the cover portion 51 and are thereby located atpositions very near to a place where the measurement is done, so thatany electric noise influences arising within these spaces is very small.In addition, since the measured values are transmitted by the interfacecircuit 30 in the form of a signal resistant to the electric noise, aninfluence of electric noise arising between the place and the dataprocessor located at the center of the automobile is also small.

Embodiment 2

FIG. 7 is a partially sectional vertical view showing a lead storagebattery for an automobile adopting a sulfuric acid concentration sensorof this embodiment. The lead storage battery is the same as that of thefirst embodiment. In FIG. 7, the same components with those of FIG. 1are attached with the same symbols.

FIG. 8 is an enlarged partially sectional vertical view of a port plug5, FIG. 9 is an enlarged sectional view taken on a line IX--IX of FIG.8, and FIG. 10 is an enlarged partial view viewed in a direction ofarrow X of FIG. 8. In these figures, the same components as those ofFIG. 2 and FIG. 3 are attached with the same symbols. The sulfuric acidconcentration sensor of this embodiment is also equipped with the quartzresonator plate 10, the oscillation circuit 20 and the interface circuit30, and is installed utilizing the conventional port plug as it is.

The holder tube 11 has an opening 11a at its side surface, and a circuitsubstrate 41 is fitted in the opening 11a. The oscillation circuit 20and the interface circuit 30 are assembled in one integrated circuit andequipped in the circuit substrate 41, and the quartz resonatorcomprising the quartz resonator plate 10 and the electrodes 12 and 13attached to both surfaces thereof is also installed in the substrate.Both circuits 20 and 30 are installed on an inside of the circuitsubstrate 41 i.e. an inside of the holder tube 11, and the quartzresonator plate 10 is so installed as to close a circular opening 41a ofthe circuit substrate 41 from its inside. The quartz resonator plate 10is secured to the opening 41a from its inside through means of anadhesive agent 16a such as an ultraviolet curing resin for example, sothat it seals the opening 41a to prevent the sulfuric acid 6 fromentering the opening. The opening 11a of the holder tube 11 is closed bythe circuit substrate 41 provided with the quartz resonator plate 10 andsealed to prevent the sulfuric acid 6 from entering the opening. Thequartz resonator plate 10 is installed at such a position that its upperend aligns with the liquid surface lower limit line 6b of the sulfuricacid 6.

The quartz resonator plate 10 only contacts the sulfuric acid 6 at itselectrode 13 side surface. The reason is as follows. If the electrodes12 and 13 located at both side surfaces are immersed in the electrolytesimultaneously, the oscillation circuit 20 becomes hard to operate sothat conditions required for the oscillation circuit 20 will becomesevere because a large admittance of electrolyte is added to an inherentadmittance of the quartz resonator plate 10.

A plating layer (not shown) of lead dioxide is formed on the electrode13 side surface of the quartz resonator plate 10 so as to cover theelectrode 13 as well. This plating layer is formed in such a way, forexample, that the quartz resonator plate 10 on which the electrode 13 isformed is subjected to electroplating with the quartz resonator plate 10immersed in lead nitrate solution and the obtained plating layer ofmetallic lead is anodically oxidized in sulfuric acid solution. A methodsuch as a vacuum evaporation, a chemical vapor phase deposition or achemical plating may be used in place of the electroplating. Theelectrode 12 installed on the inside surface side of the quartzresonator plate 10 is composed of a circular portion 12a located at acenter of the quartz resonator plate 10 and a lead portion 12b extendingfrom the circular portion 12a. The lead portion 12b is connected to alead wire 14 through a conductive adhesive agent 12c, a conductive wire12d and the conductive adhesive agent 12c. The electrode 13 installed onthe outside surface side of the quartz resonator plate 10 is alsocomposed of a circular portion 13a and a lead portion 13b, and isconnected to a lead wire 15 through a conductive adhesive agent 13c. Thelead wires 14 and 15 extend through an inside of the circuit substrate41 to be connected to the oscillation circuit 20.

The material and characteristic of the quartz resonator plate 10 are thesame as for those of the first embodiment. The circuit substrate 41 ismade of an insulating material provided with a resistance against thesulfuric acid, such as glass, ceramics and plastic etc. and having acoefficient of linear expansion as close to that of the quartz resonatorplate 10 as possible and the same as that of the holder tube 11. This isbecause of a consideration that the coefficient of linear expansion hasan influence upon the characteristic frequency of the quartz resonatorplate 10 through the deformation.

FIG. 11 is a circuit diagram showing the oscillation circuit 20 for usein this embodiment. This circuit 20 is composed of two transistors. Inthis circuit 20, the electrode 13 is in a grounded condition through alarge capacity condenser 60 in respect of alternative current. For thisreason, the oscillation circuit 20 has a characteristic that it can beactive to oscillate even if the electrode 13 is changed to be put undera voluntary direct current potential. Accordingly, even in thisembodiment, the oscillation circuit 20 is adapted to be supplied powerdirectly from the lead storage battery.

FIG. 12 is a circuit diagram showing the interface circuit 30 for use inthis embodiment. The interface circuit 30 is adapted to divide an outputfrequency of the oscillation circuit 20 into a unit of division bypassing it through a frequency divider 131, to feed an output to acounter 132, to temporarily store an output in a latch 133, to feed anoutput to a digital/analogue converter 134, and to amplify an output bya buffer amplifier 135 so as to provide an output signal. The counter132 operates on receipt of a start pulse and a stop pulse from a gatetime controller 136, and the latch 133 also operates on receipt of alatch pulse from the gate time controller 136 in the same way. Thefrequency output from the oscillation circuit 20 varies depending upon aconcentration of an electrolyte which is a subject of the measurement,however, an analogue signal proportional to the above-mentionedfrequency change can be output from the buffer amplifier 135 byselecting a dividing ratio of the frequency divider 131 and an effectivenumber of bits of the counter 132, the latch 133 and thedigital/analogue converter 134. Further detailed function of theinterface circuit 30 will be described as follows. On receipt of thestart pulse from the gate time controller 136, the counter 132 commencescounting an output frequency of the frequency divider 131. If theeffective number of bits of the counter 132 is assumed to besufficiently large for consideration of this description, the counter132 does not overflow until the stop pulse is reached after a gate time(one second, for example) has elapsed. After the counting operation isstopped by the stop pulse, bits corresponding to the above-mentionedfrequency change amount are stored in the latch 133 by a number of bitsof the required resolution and sent to the digital/analogue converter134. In this instance, since higher ordered bits above the uppermost bitto be sent to the latch 133 play no role, the effective number of bit ofthe counter 132 assumed to be sufficiently large may be practicallyreduced by an amount of meaning nothing. By doing so, the counter 132will let the unnecessary upper bits overflow during the gate time.Further, since bits at places lower than the lowermost bit to be sent tothe latch 133 play no role for the same reason, the effective number ofbits of the counter 132 may be reduced by that amount and a dividingratio of the frequency divider 131 may be increased instead of it.Moreover, the gate time may be shortened instead of using the frequencydivider 131. However, since the frequency divider 131 has a function toprolong the gate time to average a dispersion of counted value, adividing ratio is useful to some extent. Thereby, only the smallestrequired number of bits can be converted into the analogue signal by thedigital/analogue converter 134.

It is also possible to allow an indication using bar-graph by means ofcode conversion with the digital signals as they are, instead of usingthe digital/analogue converter 134.

Even in the sulfuric acid concentration sensor of this embodiment, thesulfuric acid concentration can be measured in the same operation as thefirst embodiment. In this instance, even in this embodiment, the amountof change in the characteristic frequency of the quartz resonator isaffected by the changes in the specific gravity and the coefficient ofviscosity of the sulfuric acid 6 based on the temperature change. Evenin this embodiment, however, since the quartz resonator plate 10 i.e.the quartz resonator itself, is positively provided with the temperaturecoefficient which offsets the influence based on the above temperaturechange, an error based on the temperature change in measurement of thesulfuric acid concentration is minimized.

In the sulfuric acid concentration sensor of this embodiment, theelectrode 13 at the side contacting the sulfuric acid 6 of the quartzresonator plate 10 is grounded with respect to the alternating current,however, it can operate at a voluntary potential with respect to directcurrent. The oscillation circuit 20 is active to oscillate normally sothat the sulfuric acid concentration can be measured desirably in thelead storage battery, even when it is used for any electrolyte in eachchamber with different direct current potential.

Since the holder tube 11 is made of a material having a coefficient oflinear expansion as close to that of the quartz resonator plate 10 aspossible, there is no chance for the coefficient of linear expansion toinfluence the characteristic frequency of the quartz resonator plate 10through the deformation. From this point again, the sulfuric acidconcentration can be measured correctly.

When the lead dioxide forming the positive active material softens andfalls off so as to adhere to the quartz resonator due to stirring of theelectrolyte during charging and discharging of the lead storage battery,the characteristic frequency of the quartz resonator changes to producea large error in the continual measurements of sulfuric acidconcentration up to a termination of the service life of the leadstorage battery. In the sulfuric acid concentration sensor of thisembodiment, however, there is no chance for the falling-off lead dioxideto adhere to the quartz resonator because the lead dioxide layer ispreviously formed on the electrode 13 side surface of the quartzresonator plate 10 so as to cover the electrode 13 as well. Therefore,the measurement error caused by the falling-off of lead dioxide formingthe positive active material is not produced. Incidentally, fineparticles of the lead dioxide are apt to electrostatically adhere tometal, crystal and acrylic resin etc., but lead dioxides do not adhereto each other because of their weak binding forces. Consequently, whenthe lead dioxide is previously plated onto the quartz resonator, thelead dioxide which falls off does not adhere thereto.

Further, even in the sulfuric acid concentration sensor of thisembodiment, the upper end of the quartz resonator plate 10 is positionedat the liquid surface lower limit line 6b of the sulfuric acid 6, sothat the sensor also functions as a level gauge in the same way as thefirst embodiment.

Moreover, the oscillation circuit 20 and the interface circuit 30 areincorporated in the holder tube 11 and thereby located at positionsnearer to the place where the measurement is done, than that of thefirst embodiment so that an influence of electric noise arising withinthese spaces becomes small as compared with that of the firstembodiment. In addition, since the measured values are transmitted bythe interface circuit 30 in the form of signal resistant to the electricnoise, an influence of electric noise arising between the place and thedata processor located at the center of the automobile is also small.

Furthermore, since the quartz resonator plate 10 is installed in theperpendicular position so as to cause bubbles in the sulfuric acid 6 tomove and leave a surface of the quartz resonator plate 10, that is, thebubbles are prevented from adhering to the quartz resonator plate 10, sothat the function of the quartz resonator plate 10 is securely preventedfrom being obstructed by the adhesion of bubbles.

Another embodiment

In embodiments 1 and 2, a circuit having a composition as shown in FIG.13 may be used for the interface circuit 30. In this interface circuit30, a high-frequency output signal of a frequency dividing circuit 231is not directly input to a counter 232 directly, but the signal isamplified by a buffer amplifier 237 and then coupled to a wiringarranged from a lead storage battery 241 which is a subject ofmeasurement, to a cigarette lighter receptacle 242 on a dashboard. Ahigh-frequency signal is picked up from the cigarette lighter receptacle242 by a high-frequency transformer 240 and amplified by a bufferamplifier 238 so as to be input in the counter 232. The operatingprinciple is the same as that of the circuit shown in FIG. 12, except apoint that a signal path is somewhat complicated. However, thisinterface circuit 30 is adapted to enable an operator to visuallyconfirm a final analogue output signal through means of a voltmeter 243coupled to the cigarette lighter receptacle 242. When a toroidal core isused for magnetic cores of the high-frequency transformers 239 and 240,and the signal is coupled to an existing wiring of the cigarette lighterhaving an ordinary low impedance, the only necessary procedure is to letthis wiring go through a transmission side toroidal core when installingthe circuit, and thereby an advantage is obtainable such that theinstallation does not require much trouble. The power for a receiverside circuit including the counter 232 may be supplied from thecigarette lighter receptacle 242 through a high-frequency blockingfilter.

A circuit having a composition shown in FIG. 14 may be used for theoscillation circuit 20. References 22 and 23 denote inverters.

The quartz resonator plate 10 is installed in the horizontal position inembodiment 1, and in the perpendicular position in embodiment 2. But,quartz resonator plate 10 may be installed in an inclined position bychanging a shape of a lower end portion of the holder tube 11.

The lead wires 14 and 15 may be formed by being printed with aconductive ink on an inside surface of the holder tube 11 and an insidesurface of the circuit substrate 41, by being deposited with a vacuumevaporation method and sputtering method etc., or by sticking aconductive foil using an adhesive agent. By doing so, productivity andvibration resistance can be improved.

A film surface of the electrode 13 in contact with the sulfuric acid 6may be coated with silicon dioxide or silicon nitride so as to provideit with corrosion resistance.

As described above, according to the sulfuric acid concentration sensorsof this invention, the concentration of the sulfuric acid 6 can bedetermined by obtaining the characteristic frequency of the quartzresonator oscillated in the sulfuric acid 6 forming the electrolyte ofthe lead storage battery. As a result, the charged condition or residualcapacity of the lead storage battery can be detected. For example, ifthe charging current or charging voltage is decreased when a specificgravity exceeds the specified value of 1.280, an excessive corrosion ofthe grid can be prevented, water decomposition can be controlled to aminimum and a service life of lead storage battery can be improvedconspicuously.

Since the sulfuric acid concentration sensor of this invention iscomposed of the quartz resonator immersed in the sulfuric acid 6 and theoscillation circuit 20, the composition is simple. In addition, thequartz resonator can be composed of the small quartz resonator plate 10and the electrodes 12 and 13 installed at both surfaces thereof.Therefore, the sensor can be made small and installed by utilizing theconventional port plug for example, so that the productivity can beimproved and the cost can be reduced. Accordingly, the sensor can beused practically for the lead storage battery for an automobile.

The electrode 13 at the side contacting the sulfuric acid 6 of thequartz resonator plate 10 is grounded with respect to the alternatingcurrent, so that it can operate at any direct current potential.Therefore, the oscillation circuit 20 is able to oscillate normally, sothat it is able to measure the sulfuric acid concentration ofelectrolyte in any one of several serial chambers of the lead storagebattery, each of which is at different potentials.

The quartz resonator plate 10 is installed in the perpendicular positionso that bubbles in the sulfuric acid 6 are prevented from adhering tothe quartz resonator plate 10, and the function of the quartz resonatorplate 10 is securely prevented from being obstructed by the adhesion ofany bubbles.

The quartz resonator is provided with the temperature coefficient, whichoffsets the temperature coefficient of sulfuric acid, so as to minimizean error based on the temperature change so that the characteristicfrequency can be measured correctly and the sulfuric acid concentrationcan thus be obtained exactly.

The lead dioxide layer is previously formed on the surface of the quartzresonator plate 10 contacting the sulfuric acid 6, so that the leaddioxide can be prevented from contacting the quartz resonator even whenthe lead dioxide forming the positive active material softens and fallsoff during charging and discharging of the battery. Consequently, themeasurement error caused by the falling-off of the lead dioxide formingthe positive active material is prevented.

The quartz resonator plate 10 is positioned at the liquid surface lowerlimit line 6b of the sulfuric acid 6, so that the lowering of thesulfuric acid 6 down below the lower limit line 6b can be detected bythe abnormal value. Therefore, this sensor can also function as a levelgauge.

The oscillation circuit 20 and the interface circuit 30 are incorporatedin the holder tube 11 or the cover portion 51, so that they can belocated in the vicinity of the quartz resonator. Therefore, theinfluence of electric noise can be minimized within these spaces.

The holder tube 11 and the circuit substrate 41 are made of materialshaving coefficients of linear expansion as close to that of the quartzresonator plate 10 as possible. Accordingly, the characteristicfrequency of the quartz resonator plate 10 is prevented from beingaffected by the coefficient of linear expansion through the deformation.From this point again, the sulfuric acid concentration can be measuredcorrectly.

Since the power of the oscillation circuit 20 is supplied from the leadstorage battery which is the subject of measurement, it is not necessaryto separately prepare a power source for the sensor. From this pointagain, the composition of the sensor can be simplified.

What is claimed is:
 1. In a sensor for detecting a sulfuric acidconcentration of electrolyte for a lead storage battery, a sulfuric acidconcentration sensor for the lead storage battery comprising:a quartzresonator changing its characteristic frequency in an approximatelylinear relationship according to a change of sulfuric acidconcentration, and an oscillation circuit coupled to and oscillating thequartz resonator, the quartz resonator being immersed in an electrolytesuch that a characteristic frequency of the quartz resonator duringoscillation is obtained so as to determine the sulfuric acidconcentration; wherein the quartz resonator further comprises:electrodes installed on both surfaces of the quartz resonator platerespectively, each electrode being electrically coupled to theoscillation circuit, the quartz resonator plate being so installed thatonly one side surface of the plate contacts an electrolyte, and anelectrode at the side contacting with the electrolyte being groundedthrough a condenser in respect of alternating current.
 2. A sulfuricacid concentration sensor for a lead storage battery as set forth inclaim 1, in which the quartz resonator plate forming the quartzresonator is installed in an inclined position or a perpendicularposition.
 3. A sulfuric acid concentration sensor for a lead storagebattery as set forth in claim 1, wherein the quartz resonator is formedwith a quartz resonator plate, the quartz resonator plate beingpositioned at a liquid surface lower limit line of the electrolyte tofunction as a level gauge.
 4. A sulfuric acid concentration sensor for alead storage battery as set forth in claim 1, in which the quartzresonator and the oscillation circuit are installed in a port plug ofthe lead storage battery, the port plug has a immersion portion to beimmersed in the electrolyte, the immersion portion comprises acylindrical portion having an opening, the quartz resonator is soinstalled as to close said opening to seal an inside of said cylindricalportion, and the oscillation circuit is installed in a cover portion. 5.A sulfuric acid concentration sensor for a lead storage battery as setforth in claim 1, in which the quartz resonator and the oscillationcircuit are installed in a port plug of the lead storage battery, theport plug has a immersion portion to be immersed in the electrolyte, theimmersion portion comprises a cylindrical portion having an opening, thequartz resonator and the oscillation circuit are installed on a singlesubstrate, and the single substrate is so installed that the oscillationcircuit is located in said cylindrical portion and closes said openingso as to seal an inside of said cylindrical portion.
 6. A sulfuric acidconcentration sensor for a lead storage battery as set forth in claim 1,in which a power of the oscillation circuit is supplied from a leadstorage battery a sulfuric acid concentration of which is to bemeasured.
 7. In a sensor for detecting a sulfuric acid concentration ofelectrolyte for a lead storage battery, a sulfuric acid concentrationsensor for the lead storage battery comprising:a quartz resonatorchanging its characteristic frequency in an approximately linearrelationship according to a change of sulfuric acid concentration, andan oscillation circuit coupled to and oscillating the quartz resonator,the quartz resonator being immersed in an electrolyte such that acharacteristic frequency of the quartz resonator during oscillation isobtained so as to determine the sulfuric acid concentration; and whereinthe quartz resonator has a temperature coefficient for compensating atemperature coefficient of sulfuric acid.
 8. In a sensor for detecting asulfuric acid concentration of electrolyte for a lead storage battery, asulfuric acid concentration sensor for the lead storage batterycomprising:a quartz resonator changing its characteristic frequency inan approximately linear relationship according to a change of sulfuricacid concentration, and an oscillation circuit coupled to andoscillating the quartz resonator, the quartz resonator being immersed inan electrolyte such that a characteristic frequency of the quartzresonator during oscillation is obtained so as to determine the sulfuricacid concentration; and wherein lead dioxide is plated on a surface ofthe quartz resonator at least on a side contacting the electrolyte. 9.In a sensor for detecting a sulfuric acid concentration of electrolytefor a lead storage battery, a sulfuric acid concentration sensor for thelead storage battery comprising:a quartz resonator changing itscharacteristic frequency in an approximately linear relationshipaccording to a change of sulfuric acid concentration, and an oscillationcircuit coupled to and oscillating the quartz resonator, the quartzresonator being immersed in an electrolyte such that a characteristicfrequency of the quartz resonator during oscillation is obtained so asto determine the sulfuric acid concentration; wherein the quartzresonator and the oscillation circuit are installed in a port plug ofthe lead storage battery, the port plug has a immersion portion to beimmersed in the electrolyte, the immersion portion comprises acylindrical portion having an opening, the quartz resonator is soinstalled as to close said opening to seal an inside of said cylindricalportion, and the oscillation circuit is installed in a cover portion;and wherein the immersion portion is made of a material having acoefficient of linear expansion as close as possible to that of a quartzresonator plate forming the quartz resonator.
 10. In a sensor fordetecting a sulfuric acid concentration of electrolyte for a leadstorage battery, a sulfuric acid concentration sensor for the leadstorage battery comprising:a quartz resonator changing itscharacteristic frequency in an approximately linear relationshipaccording to a change of sulfuric acid concentration, and an oscillationcircuit coupled to and oscillating the quartz resonator, the quartzresonator being immersed in an electrolyte such that a characteristicfrequency of the quartz resonator during oscillation is obtained so asto determine the sulfuric acid concentration; wherein the quartzresonator and the oscillation circuit are installed in a port plug ofthe lead storage battery, the port plug has a immersion portion to beimmersed in the electrolyte, the immersion portion comprises acylindrical portion having an opening, the quartz resonator and theoscillation circuit are installed on a single substrate, and the singlesubstrate is so installed that the oscillation circuit is located insaid cylindrical portion and closes said opening so as to seal an insideof said cylindrical portion; and wherein the substrate is made of amaterial having a coefficient of linear expansion as close to the quartzresonator plate forming the quartz resonator as possible.